Because
our eyes and minds have been hardwired by evolution to identify
patterns and relationships to help recognize the world around us,
they can be fooled by images that contain patterns that don't occur
in nature. The result is an optical illusion that makes us see
something that isn't correct. Here are several interesting and
entertaining examples:

The
gray lines between the rows of boxes are actually straight even
though they look wavy.

As
you look at the corners where the intersections of the gray lines,
the ones near the center of your field of view look white while those
further away look darker. Moving your eyes around will make the dots
flash bright and dark.

Although
the open center circle on the left looks slightly smaller than the
one on the right they are both the same size.

All
of the rows and columns of boxes are straight in the following image:

The
corner intersections are white, even though they appear a faint gray.

The
diagonal lines are all parallel

Even
though the top line looks longer than the bottom one, they are both
the same length.

All
the red boxes are the same color, even though the ones on the bottom
appear darker.

The
eternal stairway upward

Focus
on the black dot in the center of the rings and lean forward and
backward, the rings will appear to rotate in opposite directions.

Lean
close the screen and focus on the red circle as you count to 60.
Then close your eyes and you'll see a green circle where the red dot was.

This
uses a different effect than the previous illusions, in fact it
isn't an illusion at all. What happens is that as you stare at the
red dot your eye adapts to the color red. After awhile it develops a
temporary bias against red. When you then close your eyes they over
correct and you see the opposite color: green.

This
is similar to the loss of sensation we feel when something presses
against our skin with constant pressure and without movement against.
After awhile the nerves adapt to the sensation and we no longer feel
it. But, when the pressure is removed we feel a spring-back or
opposite sensation even though there isn't any stimulation occurring.

Here's
an example of how diverging lines create the illusion of objects
getting smaller:

The
following illusion gives you three illusions in one picture: the
lines look curved, the squares look different sizes and the surface
appears to bulge outward.

As
you look at the next image the outer circles will appear to rotate
clockwise while the inner circles move in the opposite direction. The
effect is enhanced by letting your eyes roam around the picture.

This picture
works in a similar manner. Moving your eyes right and left increases
the rolling-wave effect.

The
following is a double illusion: waves appear to roll from corner to
corner and the rows of squares appear warped even though they are straight:

In
the picture below it appears as if the circles are spinning as you
move your eyes around:

Focus
on the area between the red dots and lean forward until the two red
dots appear to merge into one dot. Then slowly lean back while
maintaining your focus on the red dot. At around a foot from the
screen you'll suddenly see the colored dots appear as if they were a
three dimensional image with the yellow dot the closest and the black
dots the furthest away. Off to either side you will see two more 3D
images of the same field of dots. (This may take some practice.)

This
trick can be used to create 3D images of planets. A popular one is
using two images of Jupiter taken half an hour apart. The slight
rotation of the planet shifts details on the surface so that the
image appears three dimensional.

How
it works is that your mind processes information from the focusing
angle between your two eyes to establish whether things are close or
far away. Objects near have your eyes turned in more than those far
away so your mind knows they are closer. In the case of the two
fields of dots above, although they appear to be the same, the yellow
and orange dots on the right were moved slightly to the left,
increasing the angle between the left and right fields so they look
closer. The blue and black dots on the right were moved to the right,
decreasing the angles so that they are interpreted as being further
away. (To learn how easy it is to create 3d drawings like this,
please open my HOW
TO MAKE THREE DIMENSIONAL IMAGES page.)

There
are thousands of optical illusions. Two of the best sites I've found
for viewing them are:

http://www.michaelbach.de/ot/
(many motion-related illusions where you can select the speed at
which the picture rotates)

http://psy.ritsumei.ac.jp/ric/~akitaoka/index-e.html
(another great site with hundreds of illusions)

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